Espresso machines can be broadly subclassed into four:
Super Auto – Automatically grinds, compacts and brews at the touch of a button
Semi Auto – User has to separately grind and compact ( aka tamp ) the beans. Brewing pressure comes from a pump. Volume of espresso extracted manually controlled
Fully Auto – Like Semi Auto with additional feature of automatic brew volume
Manual – User has to separately grind and compact ( aka tamp ) the beans, and pull a level to create the brewing pressure
For brewing a good cup of espresso at home or in a cafe, the semi-auto or fully-auto type machine is probably the best choice. These machines have different designs internally, including ( ordered by cost ) 1) Thermoblock, 2) Single Boiler, 3) Heat Exchanger.
Cold water is pumped through a serpentine path inside a thermoblock ( heated metal block ) to brew espresso. To produce steam, the thermoblock is heated to a high temperature and the pump is then turned on in pulses. Each pulse feeds a small amount of water into the thermoblock – the water immediately boils into steam.
The thermoblock design is relatively low cost to manufacture, so is popular in the lower priced machines. The shortcoming of this design is the water heats up during its time within the thermoblock, so the brewing temperature is dependent on how fast the water flows. Any change in the amount, grind fineness and freshness of the coffee will result in non-ideal brewing temperatures. Scale deposits within the water path in the thermoblock will result in lower brew temperatures.
The amount of steam generated by the thermoblock is also limited because not much heat is stored in the thermoblock. Any unvaporised water coming out of the thermoblock will result in the steam being “wet” making it less easy to froth the milk.
The advantage of the thermoblock is it heats up very quickly, so can the machine can be used at short notice. Overall power consumption is lower too.
For espresso brewing, water is heated inside a boiler to the required temperature. A pump pressurises the boiler and a spring operated valve opens, allowing the water to brew the coffee. Some machines have an OPV ( over pressure valve ), which ensures a consistent brewing pressure by diverting extra water from the pump back to the cold water tank.
For steaming, the same boiler is heated to a higher temperature and a steam tap is opened, allowing steam to escape from the boiler.
An improved version includes a 3 way solenoid valve which is electrically operated – this allows the pressure in the portafilter to drain immediately when the brewing is completed.
During brewing, the valve allows the water from the boiler to flow into the coffee in the basket/portafilter
When brewing is completed, the valve allows the pressurised water which has not yet flowed out of the portafilter to drain out. This allow the portafilter to be unlocked without fear of “portafilter sneeze”. Because partially brewed coffee flows out of the drain, regular maintenance by backflushing ( pulling a shot with a basket without hole and optionally filled with a cleaning agent )is required to keep it clean.
The single boiler design has moderate manufacturing cost as the number of parts is low. The size of the boiler also effects cost – the single boiler machines with larger boilers tend to cost more. The advantage of this design over the thermoblock is that the brewing temperature is more consistent – the larger the boiler the better in this regard.
Steaming performance is generally better than a thermoblock as the boiler can store a fair amount of heat when it is heated up before steaming begins. Again, size matters.
The disadvantage of this type of machine is it takes longer to heat up ( vs thermoblock ). It also takes some time to switch between brew and steam modes, which is fine when you are preparing one or two drinks but gets cumbersome if doing more.
The temperature of the boiler is typically controlled using low cost Thermostats and can cycle quite a lot. This can be overcome with methods like “Temperature Surfing” and modifications like a PID control system.
HEAT EXCHANGER ( HX )
This design elegantly overcomes the need for the boiler to be at a different temperature for brewing and steaming by having the boiler at steaming temperature all the time. For brewing, cold water is pumped into a tube running through the boiler, heating up to the correct temperature by the time it hits the coffee. To further stabilise the brewing temperature, HX machines have a large metal brewing head. This head is kept at the correct temperature by connecting it to the tube via two pipes. The circular path formed by the tube, head and two pipes creates a continuous natural convective flow of water. This is called the thermosyphon and when properly adjusted will result in very stable head temperatures.
For steaming a steam tap is opened allowing steam from the boiler to escape.
The HX machine has a higher manufacturing cost because it has more parts and typically has a large boiler and heavy brewing head – it is primarily designed for heavy duty commercial use. The advantage of the HX machine is it can produce very stable brewing temperatures. It can steam and brew at the same time ( or quick succession if you are working alone ), and because the boilers are large the steam capacity is high.
The disadvantage for the home user ( other than cost ) is it takes a long time to heat up and takes up more countertop space.
The temperature of the boiler is typically controlled using a Pressurestat, resulting in very stable boiler temperatures. For home use, the elapsed time between brewing subsequent shots is usually high so the water in the HX tube can get very hot – this can be overcome by flushing the machine before brewing. In some machines the steam power is overwhelming for new users, this can be overcome by plugging some of the holes in the steaming tip or swapping the tip.
“E61” machines have a large brewing head to help stabilise the brewing temperature and the head includes sprung chambers to reduce the ramp up of pressure during the start of the shot. This is sometimes referred to as “Pre-Infustion” and is believed to allow the coffee grounds to swell, thus filling up any gaps caused by imperfect distribution of coffee into the basket before full pressure brewing begins. Pre-Infusion makes it easier to pull consistently good shots.
There are two types of 3 way vavles used in “E61” heads – the mechanical type which is operated with a lever and the solenoid type which is operated electrically.
There are other designs which have evolved from the above three, which aim to overcome some of the shortcomings. More on those later…..